1. Field of the Invention
This invention relates to polypropylene composite stretched films having superior low-temperature heat-sealing characteristics, transparency and anti-block properties.
2. Description of the Prior Art
Crystalline polypropylene films, particularly, biaxially stretched polypropylene films, have superior characteristics in mechanical properties such as tensile strength, impact strength, modulus of elasticity, surface hardness, cold temperature resistance, etc., and optical properties such as gloss, transparency, etc. and further, odorless properties, non-toxic properties, etc.; hence they have been broadly used in the field of food packaging. However, biaxially stretched polypropylene films themselves have high heat-sealing temperatures, which results in such drawbacks that when they are subjected to heat seal at such temperatures, a large heat shrinkage occurs to reduce their sealing strength and spoil their fine appearance.
In order to overcome such drawbacks, films having a readily heat-sealable, low-melting resin laminated onto a biaxially stretched polypropylene film as a base have been broadly used. Lamination processes therefor include a process of coating a base layer with a low-melting resin in solution or in molten state, a process of laminating a film-forming low-melting resin onto a base by heating or with an adhesive, and the like processes. Further, a process has been noted in view of its high productivity, which process comprises laminating a low-melting resin onto an unstretched polypropylene film or molding a crystalline polypropylene and a low-melting resin into a film according to a co-extrusion process, followed by monoaxial or biaxial stretching. In general, when a low-melting resin of a laminated film is monoaxially or biaxially oriented, it has a higher softening point than that in the case of non-orientation and its heat-sealing temperature also becomes higher. Thus, according to the process of stretching after lamination, it is desirable to subject the stretching temperature to the melting point of the low melting component or higher to thereby keep the low-melting component at a non-oriented or nearly non-oriented state.
For the low-melting component laminated onto the biaxially stretched polypropylene film, it is desired that (i) it has a heat-sealing temperature lower than the heat shrinkage temperature of the biaxially stretched polypropylene film as the base, (ii) a high heat-sealing strength, (iii) a good adhesion to the base, and besides, (iv) a transparency to the same extent as or higher than that of the base, (v) a superior anti-block property and (vi) scratch resistance and it is non-toxic and odorless. Polyethylene and ethylene-vinyl acetate copolymers so far often used as a low-melting component are superior in heat-sealing properties but insufficient in transparency, scratch resistance and anti-block properties. On the other hand, a number of proposals of propylene copolymers having a small amount of a comonomer such as ethylene, butene-1, etc., copolymerized have been disclosed, for example, in Japanese patent publication Nos. 46-31478/1971, 49-14343/1974 and 52-30434/1978 and Japanese patent application laid-open Nos. 53-114887/1978 and 55-17542/1980. However, composite stretched films provided with all the above characteristic (i)-(vi) have not yet been obtained. Further, the above Japanese patent application laid-open No. 55-17542/1980 discloses use of crystalline propylene-.alpha.-olefin-ethylene copolymers containing 15% by weight or less of cold xylene-soluble matter, as a heat-sealing layer, but this is not economical since it is necessary for obtaining such copolymers to remove the cold xylene-soluble matter by washing the copolymers with a hydrocarbon such as liquefied propylene, n-heptane, etc.
In order to overcome these drawbacks, the present inventors have made extensive research, and as a result, have found that in propylene random copolymers as a heat-sealing layer having ethylene or ethylene and butene-1 as comonomer(s) copolymerized therein, the content of polymers having a molecular weight (abbreviated to Mw) of 6,000 or less in the copolymers is a dominating factor in the above characteristics. Further, it has been found that if the content of polymers having a Mw of 6,000 or less is in the range of 1 to 5% by weight, a good heat-sealing layer is obtained even when the content of cold xylene-soluble matter exceeds 15% by weight, and also that if the content of polymers having a Mw of 6,000 or less exceeds 5% by weight, the anti-block properties of the copolymers becomes inferior even when the content of cold xylene-soluble matter is 15% by weight or less.